1. Field of the Invention
The present invention relates to the design of a package for an integrated circuit; and, in particular, relates to the design of lead frames for a surface mount package of an integrated circuit.
2. Discussion of the Related Art
In the manufacture of a surface mount package for an integrated circuit, the solder plated leads of a lead frame in such package are trimmed to a desired length prior to mounting the package on a printed circuit board. FIG. 1 shows a prior art surface mount package 100 being mounted on a printed circuit board 150. Package 100 typically have a large number of leads, typically exceeding 30. For clarity, FIG. 1 shows only one lead 101. Printed circuit board 150 has plated on one surface a number of conductive metal traces to electrically contact the leads of package 100. In FIG. 1, lead 101 is shown contacted to conductive metal trace 102.
The leads of package 100 form a part of a lead frame to which the semiconductor die of the integrated circuit is electrically coupled. The manufacturing process of the lead frame, and the assembly process which attaches the semiconductor die, bonds the contact pads on the semiconductor die to the leads of the lead frame, and forms the encapsulation of package 100, are well known to one of ordinary skill in the art. The leads of the lead frame are each coated with a coat of solder material. This solder material can be tin, tin-lead (common "solder"), or palladium.
At one step in the assembly process of package 100, the leads of package 100 are trimmed to the desired length. FIG. 2a is a vertical section of lead 101, which shows lead 101 comprising a base metal core 120, which is typically formes by copper or a copper alloy, and a coat 121 of a solder material on base metal core 120. Lead 101 is trimmed at position 103 to the desired length, which is typically determined by the requirements of the integrated circuit's application. As shown in FIG. 2a, because of the trim, a surface 122 of base metal core 120 is exposed.
After package 100 is placed onto printed circuit board 150, heat is applied to the leads to bond the solder material of the leads to the copper traces of printed circuit board 150. This heating step is typically applied by an infra-red radiation reflow step, although a hot bar (i.e. soldering iron), or a vapor reflow step can also be used. The solder material on the leads of package 100 are liquified and wet the conductive metal traces of printed circuit board 150 to form good electrical contacts. Referring again to FIG. 1, solder material 104 is shown to bond lead 101 to metal trace 102. In general, the larger the surface area of lead 101 involved in solder wetting, from both mechanical and electrical points of view, the better the resulting contact would be. In some applications, wetting of 85% along the side surfaces of the portion of the lead contacting the printed circuit board is required.
However, as shown in FIG. 2a, trimming a lead at its desired length leaves an exposed surface not coated with the solder material. Consequently, when the lead is bonded to a conductive metal trace of a printed circuit board, wetting would not result along the exposed surface. FIG. 2b is a top view of lead 101 showing wetting occuring only on two sides of the lead. Often, this absence of wetting results in package 100 being rejected for not meeting the requirements of the intended application.